Prior to the present invention, there have existed numerous techniques of measuring the position of the liquid in an enclosed vessel. One of the most common measuring devices in this category is the MERCURY MANOMETER in which the height of the mercury column is used to counterbalance a pressure of gaseous or liquid origin. The simplest manometer is transparent with glass or plastic wall of which the height of the mercury column can be determined by visual inspection thereof and comparing to calibrated height markings along the column, since the mercury column height determination gives the magnitude of the pressure. Some corrections were needed to account for the expansion of the liquid mercury due to variable temperatures and to account for the meniscus effect between mercury and the vessel wall of the column.
Another known method uses magnets placed at an upper face of the mercury within the column and the height position of the magnet can be determined from outside of the vessel wall by the conventional magnetic flux-dection.
Other known methods use photoelectric-cell and light combinations to determine the height of the upper surface of the mercury column as registered through a transparent wall. These prior methods all have some disadvantages with regard to the accuracy of the mercury column height measurements.
Visual reading of the mercury column is tedious and is characteristically inaccurate. Measurement of column height by the magnet magnetic flux position requires complicated and expensive electrical parts and labor, and accuracy of such readings are affected by factors such as temperature, wall thickness and the like. Photoelectric-cell measuring techniques for measuring mercury column height is also complicated and dependent on transparency of the vessels column wall, the accuracy and functioning of which is hampered by particles from the mercury forming a thin coating layer along the inner face of the column above intermittent upper surface levels of the mercury.
Mercury is the heaviest liquid that can be used for these measurements, but other liquids are sometimes used if the magnitude of the pressure to be measured is low and if a very high precision is needed, beyond the precision possible with mercury.